Thoughts about React,Web Development,Performance

Published on August 17, 2022

Set Default Values for Bash Functions

Sometimes you're working on a bash function that accepts but doesn't necessarily need arguments, so it's useful to provide a default value for them.

You can specify a default value for a variable by using the following syntax:

${index:-defaultValue}

Where:

  • index is the parameter order (starting with 1 for the first parameter).
  • defaultValue is the value that the variable will be assigned if not provided.

Here's an example:

  local name="${1:-Giovanni Benussi}"

In the example above I assigned the variable name the value of the first parameter or Giovanni Benussi if not present.

Here's a function that uses the variable defined in the previous example:

say_hi() {
  local name="${1:-Giovanni Benussi}"
  echo "Hello $name"
}

You can run it and see how it behaves:

say_hi # Hello Giovanni Benussi
say_hi John # Hello John

Real World Example

I learned about this when creating a function to get the space usage in disk of some files. I needed a way to allow users to provide a list of files or default to all files in the current directory otherwise.

You can create a variable called files with a value corresponding to the first parameter or assign it the result of evaluating $(ls -A) if not provided as follows:

local files="${1:-$(ls -A)}"

Then, you can get a human readable summary of the space usage of each file with the du -hsc command:

duu() {
  local files="${1:-$(ls -A)}"
  du -hsc $files
}

Setting a default value was really useful because du doesn't include hidden files by default, so passing ls -A as a default including hidden files was really useful for my use case.

That's is, you can reach me out on Twitter if you have any question and I'll be glad to help you!